Which one of the following fields in an IPv4 header is NOT modified by a…

CONCEPT

An IPv4 router updates header fields whose meaning depends on the forwarding hop or on the packet form sent on the outgoing link. End-to-end identity fields remain those chosen by the sender unless a separate middlebox function, such as network address translation, is applied.

APPLICATION

• Time to Live (TTL) is reduced at every forwarding hop so a packet cannot circulate indefinitely.

• The IPv4 header checksum covers the header, so after TTL changes the checksum must be recomputed.

• Total Length can differ when an IPv4 router fragments a datagram to fit a smaller outgoing MTU, because each fragment carries its own length.

• The source address records the sender. Standard routing forwards the datagram without performing the separate address-translation function used by NAT.

CROSS-CHECK / CONTRAST

• Checksum and TTL are routine per-hop updates in IPv4 forwarding.

• Length changes only in the fragmentation case, but that possibility means it is not guaranteed to stay unchanged.

• Source-address rewriting identifies NAT or another special middlebox function, not ordinary IP forwarding.

Therefore, the field not modified by a typical IP router is the source address.